The present invention relates to a projection display device which permits selective implementation of two projection methods, a rear projection method and a front projection method.
FIG. 19 illustrates a prior art front projection display device comprising as described, for example, in Japanese Patent Kokai Publication No. 120192/1989.
A projector 300 is mounted on a suitable table or mount 175 facing a reflecting, front projection screen 5F, and image rays 110 from the projector 300 are projected on the screen 5F, to display an enlarged picture. An observer 400 faces the front projection screen 5F on the same side as the side on which the image rays 110 from the projector 300 are projected, and sees the displayed picture. Since the observer 400 sees the picture on the same side as the side on which the image rays 110 are projected, the method implemented by the arrangement of FIG. 19 is called a front projection method.
Details of the projector is shown in FIG. 21. As illustrated, it comprises a light source 1 comprising a lamp 120 and parabolic mirror 130 that directs white light 2 toward dichroic mirrors 14B and 14G. The projector 300 also comprises mirrors 11a, 11b and 11c, liquid crystal display panels 3R, 3G and 3B, a dichroic prism 15, a projection lens 4 and a reflecting, front projection screen 5F. The lamp 120 may be a metal halide lamp, xenon lamp, halogen lamp or other white light source. The lamp 120 is at the focal point of the parabolic mirror 130 to produce a collimated beam of white light. The dichroic mirror 14B transmits red and green light, but reflects blue light. The dichroic mirror 14G reflects green light but transmits red light. By means of the dichroic mirrors 14B and 14G, white light 2 is decomposed into three primary colors. The mirrors 11a and 11b reflect red light, the dichroic mirror 14B and the mirror 11c reflect blue light and the dichroic mirror 14G reflects green light. The reflected light is directed to the respective one of the liquid crystal display panels 3R, 3G and 3B, each of which produces a monochromatic image of the respective color, under control of an operating circuit, not shown. The three monochromatic images for the respective colors are combined by the dichroic prism 15 into a single light image of a full color. The above-described elements make up an image-forming device 200.
The lens 4 enlarges the combined image light and projects the enlarged image light 110 onto a large front projection screen 5F. In order to achieve an image of a high quality, it is necessary to control optical aberrations, so the lens 4 is in the form of a compound lens comprising a number of lens elements (not illustrated as such in the drawing). The image is focused by moving some or all of the constituent lens elements of the compound lens 4 parallel to the light axis. The image-forming device 200 and the lens 4 make up a projector 300.
FIG. 20 shows a second example of the prior art described for example in Japanese Utility Model Kokai Publication No. 115778/1989.
This example uses a rear projection method in which the observer sees the displayed picture on the side opposite to the side onto which the image rays 110 are projected.
In this example, the projector 300, similar to the one shown in FIG. 21, a first mirror 150 and second mirror 160, are placed in a cabinet 170, and an enlarged picture is displayed on a translucent, rear projection screen 5R forming part of the wall of the cabinet 170. More specifically, color image rays 110 emitted from the projector 300 follow an optical path through the lens 4, the first mirror 150, and the second mirror 160, are projected, onto the rear projection screen 5R to display an enlarged image. Because of the translucency (semi-transparency) of the screen 5R, the picture can be seen from outside the cabinet 170, i.e., on the side opposite to the side onto which the image rays 110 are projected. The mirrors 150 and 160 are used to lengthen the light path thereby to enable adequate magnification of the image with a given field angle of the lens 4, and within a limited size of the cabinet 170. Since the observer 400 sees the picture on the side opposite to the side on which the image rays 110 are projected, the method implemented by the arrangement of FIG. 20 is called a rear projection method.
The length of the optical path in the front projection method (FIG. 19) is not restricted by the size of the cabinet 170, so the light path, i.e., the projection distance (in FIG. 19 the distance between the lens 4 and the front projection screen 5F) can be made long. Compared with the rear projection method (FIG. 20) the front projection can give a greater magnification of the image displayed on the liquid crystal display panels. However, the front projection is associated with the following problems.
(1) If a person enters the space between the projector 300 and the front projection screen 5F, the projection of the image may be disturbed. Furniture, or the like may also interfere with the projection of the image.
(2) In order to project a picture of a high visibility, the front projection screen 5F is normally made of a highly reflective material. A dark room is therefore required to prevent ambient illumination from washing out dark parts of the picture, causing a loss of contrast. Thus it is difficult to read, do housework or do other chores while watching the displayed picture.
(3) If the front projection screen 5F is near a wall (which in many cases is a convenient position) it may be that, in order to maintain an appropriate distance between the projector 300 and the front projection screen 5F, the projector 300 must be disposed somewhere in the middle of the room, restricting the layout of the room.
Compared with the front projection method, the rear projection method has the following advantages:
(1) The picture is projected from the inside of the cabinet 170 onto the rear side of the rear projection screen 5R, so people or furniture will not obstruct the picture.
(2) The rear projection screen 5R may be provided with black stripes, or the rear projection screen 5R may be made of a material that selectively transmits only the spectral components of the image rays 110, thus providing good contrast in the presence of ambient illumination.
(3) Because of the relatively small size of the cabinet 170, it is easy to find a suitable place against a wall or in a corner that does not spoil the layout of the room.
In the rear projection, the picture size is limited. It can be increased by (1) lengthening the optical path or (2) enlarging the field angle of the lens 4. However the measure (1) will unavoidably lead to increase in the size of the cabinet as well, requiring more space for the installation. The measure (2) requires a large number of lens elements, increasing the price of the picture display device considerably. For obtaining large pictures, the front projection method is superior. The rear projection display devices typically have a diagonal picture size of 40-70 inches, while the front projection typically provides a 60-200 inch diagonal picture.
Thus, the front projection method gives a larger picture but has many limitations with regard to the installation (layout of the room) and illumination. The rear projection method gives a smaller picture but has relatively few limitations with regard to the installation and illumination. In the prior art the user has to choose between the two types of projection devices. Providing both of the two types, of course, is disadvantageous in terms of cost and in requiring more space for the installation.